I believe it was somewhat rare. Most were made up of wood, as metal plates were exceedingly hard to produce in that era, but not impossible..

If you look into "The army that moved a mountain", otherwise know as "The Siege of Masada" https://en.wikipedia.org/wiki/Siege_of_Masada (that's the Wiki), they used the metal plate tactics there, I know there are some good YouTube videos of it too.